Heterogeneous catalysis plays a central role in the modern chemical industry. Heterogeneous catalysts frequently comprise metals and/or metal oxides with whose surface the reactants in the reaction to be catalyzed interact. Apart from the nature of this interaction, the transport of the reactants to the active sites of the surface and the removal of the products from the surface play a critical role. In addition, the heat of reaction liberated has to be removed as quickly as possible or required heat has to be introduced.
Physical Forms of Catalysts
Owing to the wide variety of heterogeneously catalyzed reactions and the sometimes very different reaction conditions, various physical forms have become established for heterogeneous catalysts with the intention of ensuring optimal mass transfer and heat transport for the respective reaction. In beds, the catalyst is present in a disordered arrangement in the reactor, while in a packing it is installed in an aligned and ordered manner in the reactor. The use of catalysts in the form of granules, extrudates, pellets, rings or crushed material which are introduced as a bed into the reactor is most widespread. Such shaped bodies can either fill the entire reactor space or be arranged in various stages within the reactor by means of spacers. Additional mixing devices, heat exchangers or other internals can be provided at the reactor inlet or at other places in the reactor. To optimize the yield of desired product, various catalyst beds (e.g. different physical forms, catalysts having different amounts of active component or having different dopants) can be used along the flow direction. However, this way of using catalysts has the disadvantage that the beds described generally lead to a high pressure drop in the reactor. In addition, formation of channels or formation of zones having stagnating gas and/or liquid movement can easily occur, so that the catalyst is exposed to the reactants only very unevenly. Furthermore, the removal and installation of the shaped bodies which is required when the catalyst is replaced can be very complicated, for example in the case of shell-and-tube reactors or salt bath reactors having a large number of tubes.
To reduce the pressure drop, reactor internals which are intended to prevent excessively dense packing of the catalyst in the reactor have been described. WO 03/047747 describes a multichannel packing which is made of metal and in which a plurality of layers of packing having a different geometric surface area are assembled alternately and the catalyst is introduced between these. The layers of packing are selected so that the catalyst can trickle only into the channels of the packing having a low specific surface area, known as the catalyst channels. The catalyst cannot trickle into the layers having a high specific surface area for geometric reasons. This system is employed inter alia in reactive distillation. A disadvantage is that, owing to the increased porosity of the packing, a relatively large reactor volume is required and the internals offer a large metallic surface area. Additional metal in the form of the packing is required, which leads to increased materials costs. Furthermore, corrosion can occur and metal ions can be dissolved.
Catalysts which cannot be used as a bed of shaped bodies because of their small size can frequently be used by installation in permeable containers, for instance in woven mesh pouches. Such pouches comprise a wire mesh (e.g. KATAPAK® K from Sulzer AG, CH-8404 Winterthur, cf. J. P. Stringaro et al., Chemical Plants and Processing 1992, July, pp. 6 to 10) and are used as catalytically active internals in reactive distillations. Further designs are offered by Montz and CDTech and function according to a similar principle (e.g. “bales” from CDTech, Houston, USA). The use of these, packings has the disadvantage that in processes in which the catalyst is surrounded by a gas/liquid mixture, the appropriate liquid trickle density has to be adhered to exactly, which proves to be difficult in practice. This leads, when liquid flows are too high, to flow over the catalyst pouches, while a liquid flow which is too low leads to “drying out” of the catalyst.
For particular applications, catalysts can also be used in the form of monoliths having continuous channels, honeycomb or rib structure, as are described, for example, in DE-A-2709003. Monoliths offer a very low pressure drop at the expense of transverse mixing. This can in practice lead to inhomogeneous distribution of concentrations, temperatures and flow velocities and also unsatisfactory radial heat dispersion. The low ratio of catalyst (support) space mass to reactor volume is usually a disadvantage. Despite the disadvantages described, the physical form of a monolith has become established in exhaust gas catalysts because of a lack of better alternatives for motor vehicles.
There is therefore a need for shaped bodies having catalytic properties which can be used as reactor internals and have a geometry which is optimized for the respective reaction conditions. They should if possible allow high transverse mixing, i.e. equalization of concentrations and temperatures in the reactor, and only slight backmixing and sufficiently high mass transfer and heat transport and lead to only a very small pressure drop and conduct a way or introduce any positive or negative heat of reaction which may occur.
Manufacturing Process “Rapid Prototyping”
A person skilled in the art will know a manufacturing process known as “rapid prototyping” (RP) for prototype components by means of which even very intricate workpieces of virtually any geometry can be produced directly and quickly with the aid of available CAD data with virtually no manual procedures or molds. The principle of rapid prototyping is based on the layer-wise construction of components utilizing physical and/or chemical effects. Numerous processes such as selective laser sintering (SLS) or stereolithography (SLA) have become established here. The processes per se differ with regard to the material by means of which the layers are built up (polymers, resins, paper sheets, powders, etc.) and the method by means of which these materials are joined (laser, heating, binders or binder systems, etc.). The processes are described in numerous publications.
One of the rapid prototyping processes is described in EP-A0431 924 and comprises the layer-wise buildup of three-dimensional components from powder and binder. Powder which has not been bound is removed at the end and the workpiece having the desired geometry remains.
It is known from WO 2004/112988 that more than one pulverulent starting material can also be used, and US 2005/0017394 discloses the use of activators which induce curing of the binder.
In, for example, Galvanotechnik 1/2004, pp. 196 to 204, R. Knitter et al. describe ceramic reactors for use in microreactor technology. Shaping is effected by means of a rapid prototyping process chain, but with insertion of additional intermediate steps, namely 1.) producing a silicone negative mold from the original plastic model obtained by stereolithography and then 2.) using this mold for filling with a ceramic slip in low-pressure injection molding. The ceramic microreactors obtained serve, inter alia, as catalyst supports to which the actual catalysts are applied, for example as a suspension. The disadvantage of this production process is the additional process step in which the negative mold is firstly produced before final production occurs in the form of casting.
In cfi/Ber. DKG 82 (2005) no. 13, pp. 99 to 103, U. Kaufmann et al. describe the production of 3D ceramic components by means of a process which is based on the layer-wise buildup of the components in a powder bed. Possible uses of these components as implants are discussed.